Many methods for the plasma assisted physical vapor deposition have been proposed over the last 30 years and in the meantime many of them have found widespread application ( see E. Bergmann and E. Moll: plasma assisted PVD coating technologies published in Surface Coatings and Technologies, volume 37, pages 483 ff. (1989)).
Any physical vapor deposition can be viewed as a succession of three process steps, each of them stationary: Evaporation of components of the material, that will form the coating in a suitable installation called evaporator, transport of these components and eventually further gaseous components to the workpieces under molecular flow conditions or with electrostatic or electromagnetic flow management, transformation of these components into a coating with the required properties on the workpiece surfaces. Several realizations of evaporators are known and are currently in use ( see E. Bergmann and E. Moll op. cit.). In the case of physical vapor deposition of wear resistant coatings onto workpieces these realizations are either based on sputtering or on arc evaporation. ff the deposition process comprises a chemical reaction with other coating constituents brought into the coating chamber in the form of gases, the deposition is called a reactive deposition. If the coating is constituted exclusively from the material evaporated from the evaporator, the process is called a non-reactive deposition. This invention concerns both types of deposition processes.
If an arc is used for the evaporation in the evaporator, there exist the possibilities of evaporating the material on the cathode or on the anode of the arc. Since in the first case the arc connects to the cathodes only via one or several small spots, whose extension is negligible with respect to the cathode surface, one calls this process cathode spot arc evaporation. Many papers and patents describing this process have been published, since L. P. Sablev, N. P. Atamanskij, V. N. Gorbunov, J. I. Dolotow, J. I. Luzenko, V. M. Lunev and V. V. Usov showed in U.S. Pat. No. 3793179, how this phenomenon can be used for physical vapor deposition. This invention concerns an apparatus for cathode spot arc evaporation.
The practical use of the method of cathode spot arc evaporation is plagued with three problems, for which numerous remedies have been proposed over the past twenty years:
The stability problem: the stability of the arc discharge and the confinement of the spot location onto the section of the cathode surface provided for the evaporation called the target.
The droplet problem: the phase distribution of the material transported from the cathode to the workpieces, consisting mainly of atoms, ions and microdroplets.
The erosion uniformity problem: The uniformity of the target erosion. Already in U.S. Pat. No. 3793179 it was proposed to use protection shields and magnetic fields to solve the stability problem. Several further detailed embodiments that claim to solve the stability problem have since been proposed. The last is M. Belletti who proposed in EP 548032 A2 to use the magnetic field induced by the current leads. Belletti's patent contains a detailed discussion of the state of the art with respect to protection shields. The magnetic fields used are always of such shape, that the region, where the projection of the field vector onto the target surface has a relative maximum forms a closed loop. This principle was proposed by C.F. Morrison jr. in the U.S. Pat. No. 4448659. It has since been de-ailed and carried further to include magnetic fields, that vary over time or to magnetic fields that are rotationally symmetric fields rotating around an axis, that is not the axis of symmetry like in DE 4109213.
The material emitted from the cathode spot consists of atoms, ions and microdroplets. This material combined with the electrons is called the plasma beam. The distribution of the different components of the plasma beam is angle dependent. This was shown in an investigation by J. E. Daalder, that has appeared in 1976 in volume 9 of the Journal of physics D on pages 2379-2395. An important fraction of micro-deoplets in the coating formed on the workpiece makes the coating unsuited for use least decreases its performance drastically. At its origin the fractions of the different phases of the plasma beam depend on the frequency with which the cathode spots change their position on the target surface. The fraction of micro-droplets decreases with increasing frequency. Since the large electrical current leaving the cathode can be moved straightforwardly with Lorenz forces, one uses magnetic fields, with a strong vector component parallel to the target surface to channel the cathode spots into a narrow track thereby reducing the fraction of micro-droplets. This method and a corresponding embodiment was first proposed by S. Ramalingam and K. Kim in U.S. Pat. No. 4673477.
A second method of micro-droplet reduction uses deflections of the plasma beam. This method is called filtered cathode spot arc deposition. Two recent propositions for an embodiment of this method came from D. M. Sanders and S. Falabella: U.S. Pat. No. 5282944 and from N. Matentzoglu, G. Schurracher and J. Becker: DE 4125365. Matentzoglu et al. propose to insert a shield between the target surface and the workpieces. The ions are deflected electrostatically around the shield, while micro-deoplets will hit the shield and be thereby retained. U.S. Pat. No. 5282944 uses a ring shaped target (17'), housed in an annex chamber. The plasma beam is deflected by 90.degree. before it reaches the main chamber, where it will be deflected once more by 90.degree. before it reaches the workpieces (27'). The evaporator and its incorporation into the coating chamber are executed in a way, that only ions can go all the way from the target surface to the workpiece surfaces.
A third method uses an electromagnetic ionization enhancement, that ionizes the atoms and to a limited extent also the micro-droplets during the transport from the target to the workpieces. This method and a corresponding embodiment was proposed by P. E. Sathrum and B.F. Coll in EP 511153. The best method and embodiment of the state of the art combines the third and the second method. It was proposed by J. R. Treglio in U.S. Pat. No. 5317235. The cathode spot arc evaporator is realized as an annex chamber (20"). The annex chamber is linked to the main chamber by a small orifice (32"). The target surface is ring shaped and in a position, that all components must be deflected at least once in order to reach the workpieces. The deflection is provoked by a coil magnet (30").
There exist also many proposed methods and embodiments to solve the uniformity problem. They all consist in essence of electrostatic and electromagnetic means to steer the cathode spot. The electrostatic methods were invented by H. Tamagaki in EP 492592. Because of they do not yield the desired result, we will not discuss them in any detail. All magnetic embodiments further develop on the original proposition of U.S. Pat. No. 4724058 by introducing additional time modulated magnetic fields produced by coils or by moving the magnetic field in a double rotation. The state of the art is represented by H. Veltrop, B. Buil and S. Boelens in EP 283095 and by J. Reschke, W. Erbkamm. W. Nedon, R. Pochert. B. Scheffel, S. Schiller and H. Schmidt in DE 4109213.
The state of the art methods and apparatus for the solution of the 3 problems can not and do not satisfy the user of cathode spot arc evaporation, who wants to produce quality coatings at a price accepted by the market for several reasons. Not the least reason is the fact that each of the proposed embodiments and methods solves only one problem and is realized in a way, that it excludes the solution of the other problems by the means proposed for this purpose.
The embodiments for the solution of the stability problem use shields, that must be kept at an intermediate potential with respect to the cathode and anode potential. But shields at such a potential must be used in the filtered cathode spot arc deposition embodiments as guiding obstacles for the deflection of the plasma beam. Since nature accepts only one electric field at each point, shields can not on the one side enclose the cathode surfaces not facing the substrates and deflect precisely the plasma beam in the space between target and workpieces. The situation for the proposed magnetic fields is similar: To solve the stability problem U.S. Pat. No. 4673477 teaches to use magnetic fields "having portions generally parallel to the cathode" while the deflection methods require a magnetic field essentially normal to the surface. The embodiments proposed to solve the target uniformity problem also use magnetic fields, that are essentially parallel to the target surface over large portions of the target. These embodiments are therefore not compatible with filtered cathodic spot arc evaporation. Even the most skilled person can not solve this dilemma. The state of the art embodiments of filtered cathodic spot arc deposition have further severe disadvantages. A large fraction of the evaporated material that should form the coating does not reach the workpieces, typically 60-90%. This makes the embodiments double inefficient. More than half of the material, which is evaporated at great expense is wasted. More than half of the material which ought form the a precious coating sold for a good price must be removed from the shields and deflection devices at great expense. It is by no means simple for a man skilled in the art to remove thick wear and corrosion resistant hard coatings from sensitive components like coils. The above mentioned patents give no hint of how this could be achieved.
If one tries to use the state of the art methods of filtered cathode spot arc deposition or ionization enhanced cathodic spot arc deposition one discovers further unexpected disadvantages. Both embodiments do not only remove micro-droplets from the plasma beam but also the atoms, either by filtering or by ionization. It turned out, that the use synthesis of coatings from material that consists only of ions has two disadvantages: The impinging ions sputter a fraction of the deposited coating. This fraction is lost. It amounts 30-80% of the deposited coating material. The method becomes even less efficient and still more expensive. The impinging ions introduce defects in the lattice of the deposited polycrystalline coating material. Defects have an important negative impact on the electrical and optical properties of the deposited coating An excess of defects is also damaging for the mechanical properties.
The state of the art embodiments for the solution of the target uniformity problem are not satisfactory in many respects. The coils needed to time modulate the magnetic fields are difficult to integrate in high vacuum equipment. This task is rendered even more difficult by the fact, that they must be insulated from the cathode body, which is connected to a power supply. In a concrete realization the currents required to produce sufficiently large magnetic fields need a cooling circuit for the coils. Since the water cooling circuit of the cathode can not be used, one has to integrate a second independent cooling circuit. Similar mechanical engineering problems arise with the proposed complicated doubly rotation movement. Of course all these problems of mechanical design can be solved by a man skilled in the art, but the effort is gigantic. Furthermore, the use of complicated mechanical devices in vacuum and under plasma in particular is always plagued with reliability problems. In addition both state of the art embodiments for the solution of the uniformity problem do not solve the brim problem. When a circular target is used a brim forms because the erosion on the border of the target is weaker than the erosion in the central section. The target surface becomes concave upon use. This brim formation is caused by the shape of the magnetic fields used in the state of the art. They all employ a surface with a field essentially parallel to the target enclosed by two pole zones. Since the magnets must move inside the cathode body, the border sections of the target are always scanned by pole zones. Therefore this border section becomes a section not accessed by the cathode spot. A satisfactory solution for the brim problem has not been found so far.